REMOVAL OF IODIDE FROM GROUNDWATER USING SILVER CHLORIDE WHITE PAPER
Releases from the F and H Area Seepage Basins on the Savannah River Site (SRS) have caused groundwater plumes that contain a variety of contaminants. These plumes are releasing contaminants into Fourmile Branch, which is a small tributary of the Savannah River. The metallic contaminant releases to the branch are being controlled by base injection. The base injection targets cationic contaminants and was not intended to reduce the concentration of I-129 in groundwater. SRS and the regulatory agencies believe it is appropriate to investigate remedial alternatives that could reduce the I-129. The Savannah River Site Area Closures Projects (ACP) and the Savannah River National Laboratory (SRNL) are developing an innovative in situ treatment for I-129 using silver chloride (AgCl). The proposed AgCl amendment has a very small particle size and is designed to be injected into the contaminated aquifer to capture I-129. The solubility of AgI is several orders of magnitude lower than the solubility of AgCl. Thus, when I-129 comes in contact with AgCl it forms silver iodide (AgI), which is very stable and essentially insoluble in water. SRNL has been performing bench-scale column tests on the effectiveness of silver chloride to capture iodine in an aqueous solution. These initial tests evaluate silver chloride in four different particle sizes; 4-5 millimeters (standard reagent silver chloride), approximately 1 millimeters (sieved reagent silver chloride), approximately 2 micrometers (ultra fine grind without a grinding agent), and <1 micrometer (ultra fine grind with a grinding agent). The first two experiments with macro-sized particles were proof of principle tests. In these the AgCl was mechanically mixed into a portion of the soil filling the columns. The last two were to test the effectiveness of injecting particles suspended in an aqueous solution--the ability to inject the particles, their retention in the column and their effectiveness at removing dissolved iodide from solution. The amendments for these two columns were obtained from a private company specializing in ultra-fine grinding of materials. They were ground in propylene glycol because when ground in water silver metal electroplated onto the steel grinding equipment. Table 1 shows the list of columns and the amendment characteristics. The results of these column experiments demonstrate that solid silver chloride is highly effective at removing dissolved iodine from water and should be an effective amendment for removing I-129 from groundwater. Based on known groundwater chemistry and the reaction of AgCl with I- to produce AgI, the only interference will be natural dissolved stable iodine (I-127). Millings et al. (2002) reported a maximum I-127 concentration of 2.2 ug/L in groundwater from well P-27D, a background water table well near F-Area. The concentration of dissolved iodide used in these experiments is approximately 4500 times this natural concentration. Therefore, a AgCl amendment in the subsurface at the same concentration used in these experiments, might effectively remove I-129 for up to 4500 times the number of pore volumes of groundwater as observed here. This assumes homogeneous distribution of the amendment and no occlusion by other minerals in the subsurface. The only reliable way to evaluate these effects is by trial in the field. The column studies also suggest that the amendment is readily injectable into sediments, but will not be highly mobile. The fact that the effluent from Column 4 shows no iodide breakthrough after nearly 11 pore volumes indicates that some of the injected AgCl was trapped in the column. A mass balance on iodide reacted so far suggests that a minimum of 0.056 grams of silver chloride was trapped in the column to react with the iodide. This is 5.6% of the mass injected. Assume the 1-foot column represents the first foot of sediment in the subsurface into which the amendment is injected. If 5.6% of the AgCl particles are trapped in each subsequent foot of sediment through which the injectate moves, at 100 feet the concentration of particles will be 3% of the injection concentration. This is the maximum possible concentration indicated by these column studies, because far more AgCl may have been trapped in the column. Continued data collection will tell how much more. Dissolved silver concentrations from the silver chloride amendment will be low. Effluent from Column 1 was analyzed for dissolved silver and the concentrations were below the detection limit of 0.2 mg/L.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 945003
- Report Number(s):
- SRNL-STI-2008-00459; TRN: US200903%%51
- Country of Publication:
- United States
- Language:
- English
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